1. Field of the Invention
The present invention relates to a local oscillator suitable for use in a multifunction-compliant cellular phone such as a recently-developed cellular phone which complies with a plurality of different cellular phone systems and with the global positioning system (GPS) for emergency use.
2. Description of the Related Art
FIG. 5 shows a conventional frequency synthesizer used as a local oscillator. The conventional frequency synthesizer includes a voltage-controlled oscillator 51 and a phase-locked loop (PLL) circuit 52. A resonant line 51b is connected to the base of an oscillation transistor 51a in the voltage-controlled oscillator 51. The resonant line 51b is grounded at various positions via switching diodes 51c and 51d. An oscillation signal is output from the emitter of the oscillation transistor 51a and is input to the PLL circuit 52. A description of the specific structure of the PLL circuit 51 is omitted. The PLL circuit 52 includes therein a phase comparator, a reference oscillator, and the like. The input oscillation signal is compared with a reference frequency, and an error voltage is output. The error voltage is applied as a control voltage for a varactor diode 51e, thereby setting the oscillation frequency. The oscillation frequency is set by frequency data D input to the PLL circuit 51.
The frequency band can be changed by turning on/off the switching diodes 51c and 51d. For example, when the two switching diodes 51c and 51d are turned off, oscillation occurs in the lowest frequency band (approximately 1000 MHz). In order to perform oscillation in the highest frequency band (approximately 1800 MHz), the switching diode 51c is turned on, and the effective length of the resonant line is reduced. In order to perform oscillation in the intermediate frequency band (approximately 1400 MHz), the switching diode 51d is turned on. In any frequency band, the voltage-controlled oscillator 51 is controlled by the PLL circuit 52 to oscillate at predetermined step frequency intervals.
Oscillation signals in each frequency band are used as local oscillation signals for cellular phones which use different systems.
In the above-described conventional example, two switching diodes are used as means for switching the frequency band. Even when the switching diode is turned on, the switching diode has a small resistance. When high frequencies are used as in cellular phones, the effective Q of the resonant line is deteriorated. As a result, the oscillation operation becomes unstable, and the phase noise increases.